1. Field of the Invention
The present invention relates to non-impact printing apparatus for recording on a moving photoreceptor or the like and a print-head for use therewith.
2. Brief Description of the Prior Art
In the prior art as exemplified by U.S. application Ser. No. 199,981, filed May 27, 1988, printer apparatus is described which comprises a multiplicity of individually addressable and energizable point-like radiation sources, such as LED's, arranged in rows for exposing points upon a photoreceptor during movement thereof relative to and in a direction normal to the rows. Driver circuits are provided for simultaneously energizing the radiation sources responsive to respective data bit input signals applied to the driver circuits during an information line period. The print or recording head includes a support upon which are mounted chips placed end to end and upon each of which are located a group of LED's. The driver circuits are incorporated in chips and located to each side of the linear array of LED chips. The driver circuits in this apparatus include a shift register for serially reading-in data-bit signals and for driving respective LED's in accordance with the data signals.
Associated with each driver chip is a current-level controller that controls the level of current into the LED's of that group during recording. The controller comprises a current mirror having a master control circuit whose current is mirrored in slave circuits to which the LED's are connected. One advantage of this prior art printer apparatus is that current to the LED's may be changed automatically as needed, due to changes in aging or temperature of the printhead. As such changes affect the light output of the LED's, the changes to the current compensate for same so that some uniformity is provided to the recording apparatus.
In the current mirror described in this prior art, the master control circuit (of which there are 50 or more on a printhead) includes a transistor and an adjustable resistor. The resistor of each driver circuit is adjusted precisely and then permanently fixed during manufacture to provide the desired current to the respective LED's that are controlled thereby. This is a time consuming operation with attendant expense. After manufacture and during operation of the printer, automatic changing of current in a master control circuit is provided by adjusting voltage to the transistor as the resistor is now fixed. In order to change these voltages, additional circuitry is required to provide controlled analog levels of current.
It is an object of the invention to improve upon the printer apparatus of the prior art with more efficient and simplified controls, particularly for permitting adjustments to current delivered to the recording elements.